Voltage regulator circuits are used in various designs of energy saving control systems which are capable of responding to the power demands of a load by providing different output voltages. Existing voltage regulator circuits use tapped isolation transformers or auto-transformers and solid-state switches. The number of taps and the voltage range between the taps determine the output voltage range and the resolution of voltage regulation. The output voltage range is typically specified as a window centered at the nominal line voltage. For example, a voltage regulator having a total range window of 20% (from -10% to +10%) with a resolution of 2% would require 10 taps and 10 solid-state AC switches.
The switching for each tap is usually done by connecting two symmetric Silicon Controlled Rectifiers (SCRS) in an anti-parallel configuration to act as an AC switch. When the current is applied by the control system to the gates of a pair of SCRs, these SCRs conduct current to connect the power to the selected tap. When the current is removed from the gates of the SCRS, the SCRs will continue to conduct until the anode current drops below the latching current setpoint. This normally occurs, in a loaded system, when the line current crosses through zero. The problem with this type of voltage regulator is that control systems are usually not sophisticated enough to make sure that a pair of SCRs are turned off before the next tap's SCRs are gated on. If two pairs of SCRs are on at the same time, the two adjacent taps will act like a low voltage winding of a step-down transformer, and a large current will flow through the SCRs causing damage. To prevent this from occurring, in many systems, there is a time delay between removing gate current from one pair of SCRs and applying a gate current to another pair of SCRs. However, this will cause a short power interruption to the load. Such power interruption can negatively affect a load of sensitive electronic systems.
The present invention addresses the above problem by providing a voltage regulator circuit which utilizes inexpensive components to perform the voltage switching function without power interruption to the load and without high current circulating through the components during the voltage switching. Furthermore, the circuit of the present invention can be easily upgraded to provide higher resolution and/or a larger window, without substantially increasing the base cost.